In a secondary battery such as a lithium secondary battery, a separator is used to avoid contact between the positive electrode and negative electrode. Generally, the separator is prepared as a self-supported film separately from the positive electrode and negative electrode. For example, the separator is sandwiched between the positive electrode and negative electrode to make a unit structure (electrode cell) which is then coiled or laminated to constitute a battery group.
Examples of a general separator include a polyolefin resin film-based microporous film. Such a separator is produced, for example, by extrusion-molding a molten product containing a polyolefin resin composition into a sheet-like form, removing materials except for the polyolefin resin by extraction, and then, stretching the sheet.
A resin film separator needs to have mechanical strength enough to prevent itself from being broken in the production of a battery and it is therefore difficult to reduce its thickness to a certain level or less. In, particularly, a battery of the type in which many electrode cells are laminated or coiled, the number of unit electrode cells which can be accommodated per unit volume of a battery is limited by the thickness of the separator. This leads to a reduction in battery capacity. Also, the resin film separator may be poor in durability. Therefore, when the resin film is used for a secondary battery, this may bring about the disadvantage that the separator is deteriorated during the course of repeated charge/discharge reactions, leading to deteriorated cycle characteristics of the battery.